The present invention relates to a receiving device applied in a digital radio communication system, and in particular to a receiving device having a demodulating function based on orthogonal detection, and an equalizing function based on maximum likelihood sequence estimation.
FIG. 1 is a block diagram showing an example of constitution of a conventional radio device. As it is shown in the figure, in general, the receiving device comprises an orthogonal detector 110, A/D converters 120, 130, a maximum likelihood sequence estimating equalizer 150, and an oscillator 140. The orthogonal detector 110 is where a signal having gone through an orthogonal demodulation is inputted and two base band signal components orthogonal to each other are generated to be outputted to the A/D converters. The A/D converters 120 and 130 serve to convert each base band signal component inputted from the orthogonal detector 110 to a digital signal, at every sampling period, so as to output the digital signal to the maximum likelihood sequence estimating equalizer 150. The maximum likelihood sequence estimating equalizer 150 is to output an estimation signal on the basis of the digital signals inputted from the A/D converters 120 and 130. The oscillator 140 serves to oscillate and output a signal which is to be combined with the signal inputted to the orthogonal detector 110.
The orthogonal detector 110 includes a mixer 111 for combining the signal having gone through an orthogonal modulation together with the signal outputted from the oscillator 140, a Π/2 phase shifter 115 for shifting a phase of the signal from the oscillator 140 by Π/2, a mixer 112 inputting a phase-shifted signal from the Π/2 phase shifter 115 where the phase shifted signal is combined with the orthogonal-modulated signal, a filter 113 for letting only predetermined frequency component in the resultant signal from the mixer 111 pass through, so as to output an in-phase component in the base band signal, and a filter 114 for letting only the predetermined component in the resultant signal from the mixer 112 pass through, so as to output an orthogonal component in the base band signal.
The maximum likelihood sequence estimating equalizer 150 has a transmission line estimating section 151, a replica generator 152, a branch metric arithmetic section 153, and a signal sequence estimating section 154. The transmission line estimating section 151 is to calculate the in-phase component in an impulse response of a transmission line on the basis of the output from the filter 113 having been modulated to a digital signal at the A/D converter 120. The transmission line estimating section 151 also serves to calculate the orthogonal component in the impulse response of the transmission line on the basis of the output from the filter 114 having been modulated to a digital signal at the A/D converter 130. The replica generator 152 generates a replica on the basis of the in-phase component and the orthogonal component in the impulse response of the transmission line, that are being calculated at the transmission line estimating section 151. The branch metric arithmetic section 153 is to calculate a branch metric on the basis of the digitally modulated signals from the A/D converters 120 and 130 having been outputted from the filters 113 and 114, respectively, and the replica generated at the replica generator 152. The signal sequence estimating section 154 is to estimate the transmitted signal sequence in accordance with the branch metric calculated at the branch metric arithmetic section 153. In this case, the replica is a product of a convolution of a candidate signal sequence and the transmission line impulse response, which indicates an estimated value of a reception signal in case when a candidate signal is being transmitted.
In the following, operation of the conventional receiving device as constructed above will be described.
When a signal x(t)=p(t) cos xcfx89ctxe2x88x92q(t) sin xcfx89ct is inputted to the orthogonal detector 110, where xcfx89c is a carrier wave of angular frequency, the input signal x(t) and the cos xcfx89ct signal outputted from the oscillator 140 are combined at the mixer 111, which product passes through the filter 113, providing the in-phase component p(t) of the base band signal.
Furthermore, a phase of the signal cos xcfx89ct outputted from the oscillator 140 is shifted by Π/2 resulting in giving out a signal xe2x88x92sin xcfx89ct to the Π/2 phase shifter 115. Then at the mixer 112, the input signal x(t) and the signal xe2x88x92sin xcfx89ct are combined, which product passes through the filter 114, providing the orthogonal component q(t) of the base band signal.
The in-phase component p(t) of the base band signal outputted from the orthogonal detector 110 is converted into a digital signal at the A/D converter 120, at every sampling period T, providing a digital reception signal p(nT). Here, xe2x80x98Txe2x80x99 is a transmission rate of the base band signal, and xe2x80x98nxe2x80x99 is an integer.
The orthogonal component q(t) of the base band signal outputted from the orthogonal detector 110 is converted into a digital signal at the A/D converter 130, at every sampling period T, providing a digital reception signal q(nT), n being an integer.
The reception signals p(nT) and q(nT) outputted from the A/D converters 120 and 130, respectively, are inputted to the maximum likelihood sequence estimating equalizer 150. Then at the transmission line estimating section 151 provided inside the maximum likelihood sequence estimating equalizer 150, an in-phase component g(nT) of the impulse response of the transmission line is calculated on the basis of the inputted reception signal p(nT), and an orthogonal component h(nT) of the impulse response of the transmission line is calculated on the basis of the inputted reception signal q(nT).
After that, a replica pR(nT) is calculated at the replica generator 152 on the basis of the in-phase component g(nT) of the impulse response of the transmission line having been calculated by the transmission line estimating section 151. At the same time, a replica qR(nT) is also calculated at the replica generator 152 on the basis of the orthogonal component h(nT) of the impulse response of the transmission line having been calculated by the transmission line estimating section 151.
Then at the branch metric arithmetic section 153, a square of a difference between the reception signal p(nT) outputted from the A/D converter 120 and the replica pR(nT) calculated by the replica generator 152 is added together with a square of a difference between the reception signal q(nT) outputted from the A/D converter 130 and the replica qR(nT) calculated by the replica generator 152, which resultant represents a branch metric.
Next, the signal sequence estimating section 154 uses the well-known Viterbi algorithm (cf. xe2x80x9cWAVEFORM EQUALIZATION TECHNOLOGY FOR DIGITAL MOBILE COMMUNICATIONxe2x80x9d by Horikoshi et al., pp. 85-89, Triceps Publication) to estimate the transmitted signal sequence on the basis of the branch metric having been calculated by the branch metric arithmetic section 153.
In such receiving device as discussed above, there are some probable estimation errors in the signal sequence estimation by the equalizer, due to a phase difference between the in-phase component (to be referred to as Ich) and the orthogonal component (to be referred to as Qch), and due to an amplitude difference, DC offset and so forth, caused by the provision of two systems to cope with Ich and Qch respectively, as the orthogonal detector 110, the filters 113 and 114, and the A/D converters 120 and 130 are provided. As such estimation error occurs, the transmission characteristic is to deteriorate considerably. Therefore, in this conventional example, the receiving device requires a high-precision phase adjustment, gain adjustment, etc. in order to obtain a suitable transmission characteristic.
FIG. 2 is a diagram showing a receiving device as disclosed in Japanese Patent Laid-Open Publication No. 1-300611. This receiving device was designed to exclude any adjustment for eliminating the phase and amplitude differences between Ich and Qch in the receiving device shown in FIG. 1 as well as eliminating any DC offset.
As shown in FIG. 2, in this conventional example, the device comprises an oscillator 220 generating and outputting a signal cos 2Π(f0xe2x88x92B)t, a mixer 210 combining a center frequency f0 with a received input signal x(t) having a bandwidth B and the signal cos 2Π(f0xe2x88x92B)t having been outputted from the oscillator 220, a band pass filter 230, a sampling circuit 240 and an A/D converter 250 converting a signal with a bandwidth of B/2 less than f less than 3B/2 having been outputted from the mixer 210 and passed through the band pass filter 230 into a digital signal xn, a digital filter 260, a sampling circuit 270 sample-thinning the signal having been converted into a digital signal at the sampling circuit 240 and the A/D converter 250 and passed through the digital filter 260, so as to output a signal ip corresponding to Ich, and a sampling circuit 280 sample-thinning the signal having been converted into a digital signal by the sampling circuit 240 and the A/D converter 250 and passed through the digital filter 260, so as to output a signal qp corresponding to Qch.
Such receiving device as described above is improved as compared to preceding conventional devices, for the typical structure with two sets of mixers, filters and A/D converters provided for Ich and Qch, respectively, is changed to a structure with a single set of mixer, filter and A/D converter. In this way, there would be no phase difference nor amplitude difference between Ich and Qch, and as to the DC component, it is to be eliminated by the digital filter 260.
The digital filter 260 is composed of (1xe2x88x92Zxe2x88x922) filters 261, 262, H1 filter 263, and H2 filter 264. The digital filter 260 has a function of correcting the time lag between the in-phase component and the orthogonal component of the base band signal. As a result, time lags in the signals ip and qp are corrected.
In other words, the output signals ip and qp from the receiving device shown in FIG. 2 correspond to the in-phase component p(t) and the orthogonal component q(t) shown in FIG. 1, respectively. Therefore, a combination of the receiving device shown in FIG. 2 and the maximum likelihood sequence equalizer 150 shown in FIG. 1 would provide the orthogonal detecting function and the equalizing function corresponding to those shown in FIG. 1, which materializes a receiving device requiring no adjustments for eliminating any phase difference, amplitude difference and DC offset.
In the receiving device shown in FIG. 2, some of the advantages would be that the device can become adjustment-free, and can exhibit high-quality performance. Furthermore, since analog circuits such as a mixer, a filter and an A/D converter can come in a single set, the device can be miniaturized and become highly integrated.
In this receiving device, however, a digital filter as an additional circuit has to be added in order to correct any time lag between the in-phase component and the orthogonal component of the base band signal, which makes it more difficult to attempt further miniaturization or integration of the device.
It is therefore an object of the present invention to solve the above-described problems exhibited by the conventional cases, and to provide a receiving device with a high equalizing ability, which is capable of eliminating any phase difference or amplitude difference between Ich and Qch, and any DC offset, without applying any large-scale additional circuit, while such phase and amplitude differences and DC offset can be a cause to a deterioration in the transmission characteristic of the device.
In order to achieve the above objective, the receiving device of the present invention comprises: an oscillator generating a frequency different from a carrier frequency by (2n+xc2xd)xcfx890, provided that n is an integer, xcfx890=2Π/T, and T is a transmission rate of a base band signal; a mixer combining a reception signal and the signal having the frequency generated by said oscillator; a filter for filtering the signal synthesized by said mixer; an A/D converter sampling the signal passed through said filter at every T/2 so as to convert the signal into a digital signal for output; a polarity signal generator generating a polarity signal of which polarity inverts at every sample; a multiplier multiplying the signal having been outputted from said A/D converter and the polarity signal having been generated by said polarity signal generator, the resultant signal destined for output; a selector dividing up the output signal from said multiplier between a signal at time nT and a signal at time (n+xc2xd)T, which are destined for output; and a maximum likelihood sequence estimating equalizer using maximum likelihood sequence estimation for estimating a transmission signal sequence on the basis of the output signal from said selector, said maximum likelihood sequence estimating equalizer having a transmission line estimating section calculating an impulse response of the transmission line on the basis of the output signal from said selector, a replica generator calculating a replica on the basis of the impulse response of the transmission line having been calculated by said transmission line estimation section, a branch metric arithmetic section for calculating a branch metric indicating a difference between the output signal from said selector and the replica having been calculated by said replica generator, and a signal sequence estimating section using maximum likelihood sequence estimation in order to estimate the transmission signal sequence.
In accordance with the receiving device of the present invention, the filter passes through a signal component of a center frequency (2n+xc2xd)xcfx890 in the resultant signal from said mixer, and eliminates a DC component.
In accordance with the receiving device of the present invention, the maximum likelihood sequence estimating equalizer regards the sum of a square of a difference between the input signal at time nT and the replica at time nT and a square of a difference between the input signal at time (n+xc2xd)T and the replica at time (n+xc2xd)T, as a branch metric, which is used for maximum likelihood sequence estimation.
In the receiving device of the present invention having the above structure, the DC component of the reception signal is eliminated by the filter after the reception signal is being frequency-converted, simple logical circuits such as the multiplier, polarity signal generator, selector, etc. are added, and the process timing at the equalizer is changed. Therefore, no correction on the time lag between the in-phase component and the orthogonal component of the base band signal would be required, meaning that no such digital filter having been required in the conventional cases would be necessary.